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I'd add that when the sound comes from a person or some people, in "murmullo" you can't understand the words that the other person said while in "susurro" you usually can. As in "murmullo" you can't understand what he/they said that's why it's usually annoying.
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JaviFeb 8 '12 at 9:50

Generally they're not interchangeable but there are some contexts where maybe can be used as synonims (I can't think of anyone right now). As you can see below susurro makes reference to a soft or quiet sound but murmullo has more the meaning of soft and continuous background noise. The difference is really subtle.

@Javi thanks for the edit! how do you do the yellow marks?
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LauraFeb 8 '12 at 10:36

You can just put the greater than symbol (">") before the line or select the text and press the button "blockquote" which is in the edition toolbar (the buttons over the textarea in the edit mode).
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JaviFeb 8 '12 at 10:47